A valve drive of this type is known from the U.S. Pat. No. 6,155,216, which is considered to be class forming. In this document, the control timing are not changed as usual through angular adjustment of the camshaft relative to the crankshaft by means of a camshaft adjuster, but instead through changes to the contact point angle of camshaft engaging contour of a tappet relative to a cam of the camshaft.
For this purpose, the cited document proposes a tappet guide that can rotate in the internal combustion engine and that can have its rotational angle adjusted within an eccentric opening to allow the tappet to move supported in the longitudinal direction. When the tappet guide rotates about its longitudinal axis, the tappet moves on a circular track, whose radius—viewed in a projection in the tappet longitudinal direction—corresponds to the eccentricity of the opening. Because the cam engaging contours of the tappet also have a radius, the track profile of the contact point between the cam and cam engaging contours simultaneously describe a circular arc—viewed in a projection in the longitudinal axis of the camshaft. Because the track profile represents the sum of the possible contact points between the tappet and cam, the angle enclosed by the circular arc corresponds to a camshaft-related change to the control timing of the gas-exchange valve actuated by this tappet.
Consequently, this principle can be used very advantageously for a cam-selective change of the control timing of gas-exchange valves. This relates especially to internal combustion engines with only a single camshaft, on which the cams are arranged together for actuating inlet valves and outlet valves. This is because, in the cases, in which the control timing of only inlet valves or only outlet valves or inlet valves and outlet valves are to be changed independently from each other, just the use of a conventional camshaft adjuster can not support this objective.
Nevertheless, the geometrical conversion of this principle proposed in the cited document has a few serious disadvantages. First, the eccentricity of the tappet in the tappet guide increases with the maximum change of the control timing to be achieved, so that the track profile of the contact point also extending in the longitudinal direction of the camshaft leads to a considerable increase in the necessary cam width. However, the cam width is restricted to tight limits in modern and compact valve drives and especially in multi-valve per cylinder internal combustion engines, so that the proposed methods can utilize only a small portion of the geometrically possible potential for changing the control timing due to the limited eccentricity of the tappet guide. Second, the tappet frequently has a cam roller as low-friction cam engaging contours to the cam. For this reason, the tappet is to be protected from twisting about its longitudinal axis and requires a highly complex torsion lock construction due to the track curve described by it. This torsion lock follows the track curve while limiting the rotational degree of freedom of the tappet. As an example for the effort to be expended in the conversion of a torsion lock suitable for this purpose, is noted in US 2004/0065282 A1.